如何在 scala 中获取（不可变和可变）集合的列表？

发布于 2024-11-24 07:11:40 字数 1250 浏览 1 评论 0原文

我尝试构建一个（可变和不可变）集合的列表。编译器遇到麻烦，因为它无法确定该列表的类型。我一直认为我可以连接任何类型的列表，并且新列表的类型是连接列表的超类型。在下面的示例中，我定义了一些列表。您可以看到编译器给出的这些列表的类型：

val intList = List(1) //List[Int]
val stringList = List("ab") //List[java.lang.String]
val mSetList = List(mutable.Set(1, 2, 3)) //List[scala.collection.mutable.Set[Int]]
val iSetList = List(immutable.Set(1, 2, 3)) //List[scala.collection.immutable.Set[Int]]

现在我使用 ::: 运算符连接这些列表：

val intStringList = intList:::stringList //List[Any]
val intMSetList = intList:::mSetList //List[Any]
val intISetList = intList:::iSetList //List[Any]

正如预期的那样，编译器计算一个公共超类型 (List[Any ]) 两个列表。但以下内容无法编译：

val iSetmSetList = iSetList:::mSetList //type error

但如果我显式“转换”这两个列表，它就会起作用：

val setList1 : List[scala.collection.Set[Int]] = mSetList //List[scala.collection.Set[Int]]
val setList2 : List[scala.collection.Set[Int]] = iSetList // List[scala.collection.Set[Int]]
val setList = setList1:::setList2 //List[scala.collection.Set[Int]]

为什么我必须帮助编译器获取该列表的正确类型？为什么它会产生错误而不是简单地使用 List[Any] 键入它？理论上不可能计算类型 List[scala.collection.Set[Int]] 还是编译器中的一种错误？

非常感谢您的回答:-)

原文

I try to build a list of (mutable and immutable) Sets. The compiler gets into trouble as it cannot figure out the type of that list. I always thought that I can connect Lists of any types and that the type of the new List is a kind of supertype of the connected Lists. In the following example, I define some lists. You can see the types of those lists, given by the compiler:

val intList = List(1) //List[Int]
val stringList = List("ab") //List[java.lang.String]
val mSetList = List(mutable.Set(1, 2, 3)) //List[scala.collection.mutable.Set[Int]]
val iSetList = List(immutable.Set(1, 2, 3)) //List[scala.collection.immutable.Set[Int]]

Now I use the ::: operator to connect these lists:

val intStringList = intList:::stringList //List[Any]
val intMSetList = intList:::mSetList //List[Any]
val intISetList = intList:::iSetList //List[Any]

As expected, the compiler computes a common supertype (List[Any]) of both lists. But the following does not compile:

val iSetmSetList = iSetList:::mSetList //type error

But if I explicitly "cast" the two lists, it works:

val setList1 : List[scala.collection.Set[Int]] = mSetList //List[scala.collection.Set[Int]]
val setList2 : List[scala.collection.Set[Int]] = iSetList // List[scala.collection.Set[Int]]
val setList = setList1:::setList2 //List[scala.collection.Set[Int]]

Why do I have to help the compiler to get the correct type of that list? And why does it produce an error rather than simply type it with List[Any]? Is it theoretically impossible to compute the type List[scala.collection.Set[Int]] or is it a kind of bug in the compiler?

Thanks a lot for your answers :-)

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乖乖公主 2024-12-01 07:11:40

这是一个错误，并在夜间版本中修复，正如 huynhjl 怀疑的那样：

Welcome to Scala version 2.10.0.r25234-b20110705020226
  (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_24)
Type in expressions to have them evaluated.
Type :help for more information.
. . .
scala> val iSetmSetList = iSetList:::mSetList //type error
iSetmSetList: List[scala.collection.Set[Int]] = List(Set(1, 2, 3), Set(2, 1, 3))

It was a bug, and is fixed in nightly versions, as huynhjl suspected:

Welcome to Scala version 2.10.0.r25234-b20110705020226
  (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_24)
Type in expressions to have them evaluated.
Type :help for more information.
. . .
scala> val iSetmSetList = iSetList:::mSetList //type error
iSetmSetList: List[scala.collection.Set[Int]] = List(Set(1, 2, 3), Set(2, 1, 3))

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